1. Field of the Invention
This invention concerns a process of recovery of thermal energy of flue gas, more particularly in thermal power stations.
The invention also covers the device for the installation of the process and in particular a new industrial apparatus termed below "recuperator-vaporizer" for short, for the recovery of thermal energy, also termed sensible heat, of the gas resulting from the combustion of fossile fuels (natural gas, coal, lignite, . . . ) of thermal power stations, "flue gas" for short, by its transfer to heater condensates which condensates are vaporized.
By thermal power station, is intended any installation for transformation of thermal energy into mechanical energy by means of condensable fluid making a thermodynamic cycle, in particular fossil fuel thermal power stations.
2. Description of the Prior Art
The known state of the prior technique or art is represented in FIGS. 1 and 2 of the drawing.
FIG. 1 represents the diagram of a conventional thermodynamic cycle of a power station, with the turbine driving an electrical generator not shown. The water cycle comprises passing water to boiler (B) where it is successively heated to the boil, vaporized and superheated. It subsequently expands in the "high pressure" turbine (HPT) and next it is resuperheated (RS). It then expands in the "low pressure" turbine (LPT), after which it condenses in condenser (C), whence water extraction pump (EP) of the condenser conveys it in the successive "low pressure" heaters (H1, H2, H3 and H4) where it is heated by the steam extracted from the turbine (LPT).
The water then reaches the degasser (DG), next the feed tank (FT), at the outlet of which the feed pump (FP) brings it to the high pressure of the cycle and redelivers it to the boiler after passing in the three "high pressure" successive heaters (H6, H7 and H8).
The part of the cycle which is covered by the present invention is essentially the "low pressure" heaters. Patent No. EP 0 032 641 represents in its FIGS. 1 and 3 such a part of the cycle.
The invention also concerns the flues gas cycle represented by FIG. 2. Atmospheric air (A), for example at 20.degree. C., enters the air preheater (AP) where it is preheated, for example to 285.degree. C., by the flue gas. It then enters in the boiler (B) where it burns the fuel and leaves in the form of flue gas (FG), for example at 330.degree. C., to enter the air preheater. This flue gas is cooled by the atmospheric air (A) which it heats and then is discharged to the atmosphere.
Corresponding with the preceding temperatures given as an example, the temperature of the discharged flue gas is 120.degree. C. In practice, the temperature of the flue gas discharged to the atmosphere is between 115.degree. C. and 185.degree. C. An appreciable quantity of energy is thus lost, corresponding to the sensible heat of the flue gas.
The temperature of the flue gas at the outlet of the preheater is conditioned by several factors. As the air preheater is a gas/gas exchanger, it has poorer exchange coefficients than those where a fluid is liquid or those where a fluid changes state. Taking into account the large variations of air temperature going through the exchanger, of the order of 150 to 250 degrees Celsius, the air preheater is a large apparatus of a prohibitive cost if it had still to modify (respectively raise and lower) much more the temperature of the air flows to substantially reduce the energy loss.
However, it is the taking into consideration of the water condensation phenomenon, as the water may be acidified by the oxides of sulphur resulting from the combustion of sulphureous fuels, in the flue gas which will determine the minimum permissible temperature for the flue gas. This condensation is fundamentally avoided because of the dangers of corrosion and of the cost of the preheater if it had to be made in special materials resistant to sulphuric acid. In air preheaters, the condensation phenomenon on cold walls is stimulated by the low temperature of the ambient air entering therein, the temperature of which is markedly below the dew point of the flue gas. The dew point is a function of the humidity of the flue gas, which depends on the hydrogen content of the fuel. In many cases it is between 60.degree. and 70.degree. C. The presence of sulphur oxides however raises this dew point. Taking into account this condensation phenomenon on cold walls, the average temperature of the outgoing flue gas must be markedly above the theoretical dew point.
Another element to be taken into consideration is the draught of the stack discharging the flue gas to the atmosphere. A high temperature is favorable to the draught of the stack (reduction of the air blowing power) and to the dispersion of the flue gas in the atmosphere.
On the other hand, constraints concerning the protection of the environment currently lead to desulphurize (partly) the flue gas, essentially to reduce the acidity of rain.
The standard processes of desulphurization consist in scrubbing the flue gas with chemical solutions. It is desirable for this operation that the flue gas should not enter at an excessively high temperature (particularly at the air heater outlets) in the desulphurization installation. Patent No. FR 2 534 150 describes a desulphurization installation of flue gas where this gas is cooled beforehand to about 80.degree. C. in a heat exchanger which, after desulphurization, reheats it to convey it to the stack. But the heating of the desulphurized flue gas is not absolutely necessary since the stack and its blower can be dimensioned for fairly cool gases. It is also possible to discharge this flue gas through natural draught cooling towers, which can disperse the flue gas in the atmosphere much more advantageously with respect to the protection of environment than the conventional stacks of power stations. Patent DE 24 53 488 concerns the discharge of cleaned flue gas by the natural draught cooling towers.